The present invention relates to gaskets and, more particularly, to a gasket that forms a seal under less stress than required with existing gaskets.
A wide variety of gaskets are known for use in sealing applications. Expanded polytetrafluoroethylene (PTFE) is widely used today as a gasket material. As disclosed in U.S. Pat. No. 3,953,566 to Gore, this material has numerous properties making it highly desirable as a gasket. These properties include being readily compressible and conformable, being chemically resistant, having relatively high strength, and being far less prone to creep and loss of sealing pressure than non-expanded full density PTFE alone.
In many sealing applications, the gasket is used to seal the junction between flanges, such as between pipes. In such applications, expanded PTFE is a desirable material for the gaskets because the expanded PTFE gasket can be placed between the flanges, and the flanges can then be pressed together with the application of force, such as by tightening of bolts. This application of force compresses the expanded PTFE. As the expanded PTFE is compressed, its initial pore volume is reduced, thus densifying the expanded PTFE. Particularly with metal-to-metal flanges, it is possible to apply sufficient force (or xe2x80x9cstressxe2x80x9d) to the flanges to fully densify the expanded PTFE. Thus, in at least part of the expanded PTFE gasket, the pore volume is reduced to substantially zero, such that a fluid contained within the pipes is prevented from leaking between the flanges by the densified, non-porous PTFE gasket, which seals the flanges.
In many applications, particularly when harsh chemicals are used which would readily break down the metal or the metal could contaminate the chemical which is being transported or housed, it is common to use glass-lined steel, glass, or fiberglass reinforced plastic (xe2x80x9cFRPxe2x80x9d) piping and vessels. Because this equipment is so often used with extremely harsh chemicals, there is great desire to use PTFE gaskets to seal the connecting flanges of this equipment because of the well known extraordinary chemical resistance of PTFE. Unfortunately, non-expanded full density PTFE gaskets are generally not conformable enough to effectively seal this type of equipment. In the case of glass-lined steel flanges, although there is a relatively smooth finish, there is often a large amount of unevenness or lack of flatness associated with the flanges. This unevenness or lack of flatness requires the gasket to have to conform to large variations around the perimeter as well as between the internal and external diameter of the flange in order to create an effective seal. Thus, a non-expanded full density PTFE gasket is not conformable enough to seal many of these applications.
Because expanded PTFE is so conformable, it would be desirable to use expanded PTFE to seal these commonly uneven flanges. Unfortunately, in many of these applications it is not possible to apply sufficient force to the flanges to create enough gasket stress to fully densify the expanded PTFE gasket to create an effective seal. For example, glass-lined steel piping flanges, glass flanges, or FRP piping flanges may deform, fracture, or break upon the application of a high amount of stress. Thus, in these applications, an expanded PTFE gasket may not be completely densified to reach a non-porous state, and therefore does not become leak proof, because the maximum stress that can be applied to the flanges without breaking them is not sufficient to so densify the gasket.
In many cases, it is not only necessary to be able to seal the actual fluid being housed or transported, but it is additionally necessary for the gasket to provide an air tight seal which can pass what is commonly known in the industry as a xe2x80x9cbubble testxe2x80x9d. It is common to run this type of test as a pre-start-up qualifying test for checking for leaks in piping systems before allowing the system to be used in production carrying the actual fluid for which it was intended. In this test, the gasketed piping systems are pressurized with air and then sprayed with soapy water. The pipe and flange assemblies are visually checked for bubbles appearing in the soapy water indicating air leakage. All leakage sites must be eliminated to pass the bubble test.
Thus, what has been desired for many years is an easy-to-use highly chemically resistant gasket, which can effectively conform and provide an air tight seal for this equipment with the low loads or stresses that are available to create the seal.
There have been many attempts to provide a gasket that can effectively seal these difficult applications. Most of these attempts involve a two-piece gasket. These gaskets are commonly referred to as envelope gaskets. In most envelope gaskets, an outer envelope of PTFE is formed and is then separately filled with a more compressible filler material such as compressed asbestos or other felted gasket filler, an elastomer or plastic material, or a corrugated ring of metal, usually stainless steel. The basic concept is the PTFE jackets for the envelope gaskets provide chemical resistance while conformability is provided by the filler material.
Unfortunately, as explained in U.S. Pat. No. 4,900,629 to Pitolaj, envelope gaskets are subject to a number of disadvantages. The envelope jacket often will fold over on itself during installation of the gasket, thereby creating creases in the gasket that cause leaks. Also, there may be pin hole leaks in the envelope itself, causing corrosive material to attack the envelope filler resulting in degradation of the filler. When the filler degrades, sealing stress can be diminished, causing a leak to occur. Another problem, which can result, is that the degraded filler material can contaminate the fluids that were contained within the pipe or vessel. In some instances, the envelope jacket of PTFE will separate from the conformable filler material and ripples or folds may occur merely from stretching the envelope over the filler, again causing leaks to occur. Also, if uneven flange torquing occurs, the jacket may become overstressed and burst, once again allowing the corrosive material to attack the filler resulting in degradation of the filler and loss of the seal. Another problem is that these envelope gaskets are also subject to cold flow or creep, which requires periodic bolt retorquing.
In U.S. Pat. No. 5,195,759 to Nicholson, an envelope gasket is employed with a PTFE envelope within which is an elaborate metal filling consisting of wound or nested turns of thin metal strips perforated to provide resilience in the direction of their width. Individual turns can move or collapse to different extents, thereby accommodating lack of flatness of the surfaces to be sealed. Turns of fluid-impervious material may be distributed among the turns of the perforated strips. Although the gasket has some advantages, it still suffers from many of the disadvantages mentioned above associated with envelope gaskets, such as chemical attack of the metal filling under certain conditions.
In U.S. Pat. No. 5,558,347 to Nicholson, a gasket is disclosed comprising an envelope of chemically resistant PTFE and a metallic packing ring within the envelope is shaped to form cells. The cells may be filled with an inert gas under pressure so that increased loads on the gasket may be cushioned. Although this gasket also has some advantages, it still suffers from many of the same disadvantages mentioned above associated with envelope gaskets.
In Japanese Laid-Open Patent Application Number 4-331876 to Ueda et al., another envelope (jacket) gasket is proposed in which the outer periphery of a core composed of low-density porous PTFE that has been fibrillated (expanded) and has a density of 1.8 g/cc or less is covered with a sheath composed of high-density sintered PTFE. Although this gasket has the benefit of being 100% PTFE, and therefore does not suffer the chemical attack problems resulting from pinhole leaks in the outer envelope, it can still suffer from the aforementioned problem of the outer envelope or jacket folding over on itself during installation of the gasket, thereby creating creases in the gasket that cause leaks. It can also suffer from the aforementioned problem of the envelope jacket of PTFE separating from the conformable filler material creating ripples or folds that can result in leaks. Another problem with this gasket is that there is not a tight fitting contact between the envelope jacket and the inner porous PTFE core along the inner diameter of the gasket, thus leaving the envelope jacket without a backing in this area, and therefore more susceptible to damage during installation and while in use.
As mentioned in U.S. Pat. No. 4,900,629 to Pitolaj, in an attempt to rectify some of the problems associated with envelope gaskets, a homogeneous PTFE gasketing material filled with microbubbles (i.e., glass microballoons) was developed. This material, as illustrated by Garlock Style 3504 gasketing manufactured by Garlock, Inc. of Palmyra, N.Y., uses glass microballoons to impart compressibility (25% to 35%) to a PTFE binder, thereby providing a more deformable gasket without the disadvantages experienced by multiple component gaskets. This homogeneous PTFE/microballoon gasketing material exhibits enhanced compressibility and sealing characteristics due to the incorporation of microballoons, while maintaining the resistance to chemicals and the enhanced temperature characteristics provided by PTFE. However, the addition of the microballoons to the PTFE lowers the tensile strength properties that would be provided by pure PTFE gasketing. Plus, this gasket does not enjoy some of the aforementioned advantages that expanded PTFE has over non-expanded PTFE.
In U.S. Pat. No. 4,900,629 to Pitolaj, an attempt is made to overcome the inherent weakness of the homogeneous PTFE/microballoon gasket by loading more microballoons in the gasket surface layers, while leaving an unfilled PTFE center section. The microballoon filled layers are each formed to be within the range of from 20-25% of the overall thickness of the resultant gasket material, while the central PTFE section is within the range of from 50-60% of the overall gasket thickness. As explained in this patent, these ratios are important because if the outer surface layers are each formed to be below 20% of the overall gasket thickness, the finished composite sheet loses compressibility, while if they are formed to be above 25%, creep resistance and tensile strength are sacrificed in the finished product. Although this gasket is an improvement upon the homogeneously loaded microballoon gasket, and avoids the problems associated with envelope gaskets, it still does not adequately solve the problems of many applications. It is still left trying to trade off compressibility with creep resistance and tensile strength. This gasket also does not enjoy some of the aforementioned advantages of expanded PTFE compared to non-expanded PTFE.
In another attempt to rectify the two-piece nature problems associated with envelope gaskets, in U.S. Pat. No. 5,112,664 to Waterland, a unitary shielded gasket assembly is provided for use in corrosive environments having a synthetic rubber gasket as a core and a shielding material of expanded high density PTFE with an adhesive on at least one surface of the shielding material at least partially enveloping the surface of the core gasket. This gasket does not suffer from the wrinkles and folds that can result from a two-piece envelope gasket; however, it still suffers from the inherent problem of chemical attack problems resulting from pinhole leaks in the outer sheath.
In still yet another attempt to rectify the problems associated with envelope gaskets, in European Patent Application No. EP 0 736 710 Al, an annular gasket composed of porous PTFE for sanitary piping is proposed in which the surface layer of a gasket inner part directly contacting with sealed fluid is formed as a pore-free fused solidified layer. It is stated that the osmotic leak from the gasket inner part is prevented by the pore-free fused solidified layer formed in the gasket inner part although the gasket is composed of a porous material. Moreover, it is stated that since the fused solidified layer is formed only on the surface layer of the gasket inner part, the intrinsic properties of porous PTFE such as flexibility and affinity are not spoiled. This gasket enjoys the benefits associated with a pure PTFE gasket; however, it can be difficult to attain a robust pore-free fused solidified layer that adequately resists permeation under stress. Furthermore, because of the rounded convex nature of the flanges of glass-lined steel, in many cases there is a ready leak path between the pore-free fused solidified layer formed in the gasket inner part of the gasket and where the flange contacts the gasket. This leak path is shown in FIG. 20. This figure shows a side cross-sectional view of a gasketed flange assembly 90 of two conventional glass-lined steel flanges 96 which have the rounded convex mating edges 95 which contact the gasket 91 on part of its top and bottom surfaces 94. It can be seen that if only the surface layer of the internal diameter 93 of the gasket 91 is impermeable to the contained fluid, there is a ready leak path 92 through that exposed part of the gasket 91 which is not impermeable to the fluid.
It would be desirable to provide a unitary, conformable, creep resistant, high strength, chemically resistant gasket that can seal openings, especially glass-lined steel flanges, upon the application of a relatively low stress. It is therefore a purpose of the present invention to provide a unitary expanded PTFE gasket that provides a substantially air impermeable seal only upon the application of a low stress.
The present invention provides a multilayer, unitary gasket including at least one inner layer of expanded PTFE disposed between a first substantially air impermeable outer layer and a second substantially air impermeable outer layer, and a substantially air impermeable region bridging the first and second substantially air impermeable layers.
In another aspect, the present invention provides a multilayer, unitary gasket including an annular ring having a top surface, a bottom surface, an inside edge, an outside edge and an axis; a first substantially air impermeable layer disposed on the top surface; a second substantially air impermeable layer disposed on the bottom surface; at least one layer of expanded PTFE disposed between the first and second substantially air impermeable layers; and a substantially air impermeable region bridging the first and second substantially air impermeable layers; wherein all of the layers are oriented substantially perpendicular to the axis.
In another aspect, the present invention provides an annular gasket having an inner perimeter, an outer perimeter, a top surface, and a bottom surface including a first chamber of expanded PTFE disposed adjacent to the inner perimeter having a first air impermeable top layer on the top surface and a first air impermeable bottom layer on the bottom surface; a second chamber of expanded PTFE disposed adjacent to the outer perimeter having a second air impermeable top layer on the top surface and a second air impermeable bottom layer on the bottom surface; and a substantially air impermeable region disposed between first and second chambers.
In still another aspect, the present invention provides an annular gasket having an inner perimeter, an outer perimeter, a top surface, and a bottom surface with a first chamber of expanded PTFE disposed adjacent to the inner perimeter having a first top portion on the top surface and a first bottom portion on the bottom surface, wherein the first top portion and the first bottom portion are less permeable to air than the expanded PTFE of the first chamber; a second chamber of expanded PTFE disposed adjacent to the outer perimeter having a second top portion on the top surface and a second bottom portion on the bottom surface, wherein the second top portion and the second bottom portion are less permeable to air than the expanded PTFE of the second chamber; and a region disposed between the first and second chambers, the region being less permeable to air than the expanded PTFE of the first and second chambers. In alternative embodiments, the region may be disposed on either the inner or outer perimeter.